The invention relates to an input circuit and a method for its operation, wherein the input delay of a time delay element of the input circuit can be modified during operation of the input circuit.
An input circuit of the generic type and a corresponding operating method are known in the art. The prior-art input circuit has an input, a voltage divider with a center tap, a time-delay element, a control device and means for the high and/or low level connection. A first pole of the voltage divider forms the input of the input circuit. A signal input of the time delay element is located at the center tap of the voltage divider, and an output of the time delay element is connected to the control device. The means for the high level and low level connection are controlled by the control device and act on the signal input.
The signal applied to the input of the input circuit is hereinafter referred to as the process signal. It is supplied, for example, by a limit switch of a technical process. The input circuit is thus provided to receive digital or analog signals from a technical process controlled and/or monitored, for example, by an automation device such as a programmable controller. The input circuit can be integrated into an input module, which can be combined, particularly in a modular type of construction, with a central processing unit of the programmable controller.
The time-delay element used in the prior-art input circuit is an RC element. The delay time is determined by the dimensioning of the resistor/capacitor combination of the RC element. The high level and low level connection is provided to test the input circuit. When the high level connection is activated, a defined high signal based on a reference potential is applied to the signal input, and when the low level connection is activated, a defined low signal based on the ground potential is applied to the signal input.
One disadvantage of this prior-art input circuit and the corresponding operating method, however, is that the delay time, which is also referred to as the input delay, is constant.
Thus, one object of the present invention is to provide an input circuit and a method for its operation, such that the input delay of the time delay element can be modified during operation of the input circuit.